An IC tester is already realized for analyzing the failure of a DUT which is irradiated by a sweeping electric charged particle beam and measuring the quantity of the secondary electrons which are generated at each irradiated point by the beam. The IC tester obtains these measured parameters as an electrical signal of distribution voltage potential on the DUT and displays a corresponding voltage contrast image.
FIG. 13 shows an outline of construction for a kind of conventional IC tester. A numeral 100 in the figure shows an overall structure of an IC analysis system. The IC analysis system 100 can be classified into two blocks which comprises a test pattern generator 200 and an electric charged particle beam system 300.
The test pattern generator 200 provides a pattern signal to the DUT which is set in the ion beam tester 300. A conventional test pattern generator 200 comprises a start switch 201 which can start generation of a test pattern and, a stop switch 202 which can stop generation of the test pattern. The pattern generator 200 further includes a stop pattern setting means 203 which can stopped a renewal of the test pattern when a specified test pattern is generated, and a pattern hold means 204 which can stop renewal of the test pattern by detecting generation of the test pattern which is set in the stop pattern setting means 203. A stop signal occurrence means 205 is provided which generates a signal indicating a stop of renewal of the test pattern. Thus, the test pattern generator 200 can execute start and stop control of generation of the test pattern signal and stop of the renewal operation of the test pattern.
An ion beam tester 300 comprises a column 301 which irradiates ion beam to the DUT, a chamber 302 which keeps the DUT in a vacuum environment and is located under the column 301, a stage 303 which moves a position of the DUT for X-Y direction and is located in the chamber 302. The ion beam tester 300 further includes a sensor 304 which measures quantity of the secondary electrons generated by the DUT, and an image data acquisition device 305 which obtains image data based on an electrical signal detected by the sensor 304. A monitor 306 is provided which displays a potential contrast image which is derived from the image data processed by the image data acquisition device 305. A column control unit 307 is further provided which controls emission of the ion beam, its quantity of the emission (current value), acceleration voltage, scanning speed and scanning area.
When the pattern hold means 204 detects generation of the test pattern which is specified by the stop pattern setting means 203, the test pattern generator 200 temporarily stops a renewal operation for the test pattern and continuously outputs the test pattern which is specified by the stop pattern setting means 203. The image data acquisition device 305 and the column control unit 307 are provided with a stop signal from the stop signal occurrence means 205 which indicates a stop of the renewal operation of the test pattern. The column control unit 307 controls the ion beam emission after receiving the stop signal and the image data acquisition device 305 starts obtaining the image data.
A detection procedure for failure part of the IC is performed by setting the test pattern which has detected a failure device tested by the IC tester beforehand to the stop pattern setting means 203. When the pattern hold means 204 detects generation of the failure detecting test pattern which is specified in the stop pattern setting means 203, the test pattern generator 200 temporarily stops the renewal operation for the test pattern and continuously outputs the same test pattern. The column control unit 307 executes the emission control of the ion beam after received the stop signal and, the image data acquisition device 305 starts obtaining the image data. Above mentioned procedure for obtaining the image data is executed for both good devices and failure devices and, the image data acquisition device 305 compares, operates and, determines a failure part which is a part showing a difference between the good device and the failure device.
In this kind of conventional IC tester, a stopping time of the test pattern is set slightly longer than a time required for obtaining the image data. Because of this reason, when obtaining conditions of the image data, it is necessary to change the stopping time of the test pattern, which lowers an operability of the IC tester.
Namely, the acceleration voltage, the scanning speed, the scanning area and the number of scan of the ion beam must be specified for obtaining the image data. And when these specified parameters are changed, a time required for obtaining the image data will also be changed. For this reason, if the conditions for obtaining the image data have to be changed, then the stop time of the test pattern must also be changed. Therefore, both the test pattern generator 200 and the ion beam tester 300 must be changed in their data, which requires complicated operational procedures.
On the other hand, obtaining the condition of the image data is necessary to change for various purpose of test. In particular, if the DUT is an IC chip which is covered by an insulation film on its surface as a protection layer, electrical potentials of conductor wires which is buried under such an insulation film have to be observed. However, it is difficult to obtain an electrical potential distribution as a contrast image of an electrical potential of conductor wire in the IC whose surface is covered by the insulation film. Namely, the ion beam irradiated to the surface of the insulation film then the electrical potential distribution will be faded out in proportion to the irradiation time of the ion beam by the storage of the charge on the insulation film. Therefore, there is a problem in that an accurate electrical potential image will not be obtained.
FIG. 14 shows this situation. FIG. 14A shows the electrical potential contrast image in case of a logical L, logical H, logical L and logical H are respectively provided to conductors L1, L2, L3 and L4 existing under the insulation film. The logical L of the electrical potential (nearby 0 volt or negative voltage) is provided then the electrical potential contrast image is displayed by white color (detected secondary electron by a sensor 304 is large) as shown in FIG. 14. The logical H of the electrical potential (higher positive voltage than 0 volt) is provided then the electrical potential contrast image is displayed by black color (detected secondary electron by a sensor 304 is small). Here, an insulation substrate PB becomes an intermediate level between the electrical potentials of the logical L and the logical H, and is displayed by gray color.
FIG. 14B shows a condition just after (0.1 to 0.3 second after) the end of the irradiation and scanning of the ion beam. The electrical potential contrast goes down rapidly and disappears after few seconds from the start of irradiation and its scanning of the ion beam as shown in FIG. 14C. Accordingly, obtaining the image data is effective only in the condition as shown in FIG. 14A. Majority of the image data cannot be obtained because a time of existing of the electrical potential contrast is too short and, it is difficult to obtain a clear image by obtaining image data at one time.
There need frequent changes in the conditions for obtaining the image data (i.e., scanning area of the ion beam, the ion beam current and etc.) because the above mention phenomenon of deterioration of the electrical potential contrast exists. Accordingly, specifying the stop time of the test pattern needs to change every time when the conditions for obtaining the image data is changed thus, the operability of the IC tester is deteriorated.